1. Field of the Invention
The present invention relates to a multilayer optical recording medium with a plurality of stacked recording and reading layers from which information can be read by light irradiation, and in particular, to a technique for enhancing the quality of a reading signal.
2. Description of the Related Art
In the field of optical recording media, recording density has been increased by shortening the wavelengths of laser light sources or by increasing the numerical apertures of optical systems. However, efforts by light sources or optical systems have reached their limits. Accordingly, a volumetric recording system in which information is multiply recorded in the direction of an optical axis has been desired in order to increase recording capacity further. With reference, for example, to optical recording media in accordance with the standards for Blu-ray Disc (BD), those with two recording and reading layers have already been on the market, and those with three or four recording and reading layers wait for standardization at the time of filing of the present application. Further, multilayer optical recording media with 20 recording and reading layers dedicated to reading, and multilayer optical recording media with 10 to 16 recording and reading layers usable for recording and reading have been demonstrated in the stage of experiment and research.
Multilayer optical recording media may suffer from mixing a signal of other recording and reading layers into a target recording and reading layer, or leakage of noise generated by the effect of other recording and reading layers into a target recording and reading layer during the reading of information from the target recording and reading layer. Such mixing or leakage problems generally referred to as crosstalk result in degradation of a servo signal or a recording signal.
The crosstalk includes two types including interlayer crosstalk and confocal crosstalk. The interlayer crosstalk is a phenomenon produced by leakage of light reflected off a recording and reading layer next to a recording and reading layer being read into reading light. Accordingly, the interlayer crosstalk is always a matter of concern in multilayer optical recording media with two or more recording and reading layers. The interlayer crosstalk is reduced by increasing an interlayer distance.
The confocal crosstalk is specific to multilayer optical recording media with three or more recording and reading layers. The confocal crosstalk is a phenomenon produced by coincidence in optical path length between primary reading light reflected off a recording and reading layer being read only once, and stray light reflected off a different recording and reading layer a plurality of times.
Principles of generation of the confocal crosstalk are described with reference to FIGS. 15 to 18. In a multilayer optical recording medium 40 shown in FIG. 15, a beam 70 focused on an L0 recording and reading layer 40d for reading or recording is split into a plurality of optical beams due to semi-light-transmitting properties of recording and reading layers. FIG. 16 shows a phenomenon where a beam 71 branching off from a beam targeted for recording and reading to and from an L0 recording and reading layer 40d is reflected off an L1 recording and reading layer 40c and is focused on an L2 recording and reading layer 40b, and the resulting reflected light is detected after being reflected off the L1 recording and reading layer 40c again.
FIG. 17 shows a phenomenon where a beam 72 branching off from a beam targeted for recording and reading to and from an L0 recording and reading layer 40d is reflected off an L2 recording and reading layer 40b and is focused on a light incident surface 40z, and the resulting reflected light is detected after being reflected off the L2 recording and reading layer 40b again. This stray light phenomenon is called rear focus light of a light incident surface. FIG. 18 shows a phenomenon where a beam 73 branching off a beam targeted for recording and reading to and from an L0 recording and reading layer 40d is not focused on a different recording and reading layer, but is detected after being reflected off L1, L3 and L2 recording and reading layers 40c, 40a and 40b in this order.
The light amounts of the beams 71 to 73 as stray light are smaller than that of the beam 70. However, the beams 71 to 73 enter a photodetector with the same optical path length and with the same radius of light flux, generating influential interference. Accordingly, the amount of light received by the photodetector can vary largely in response to the minute change of an interlayer thickness, making it difficult to detect a stable signal. Meanwhile, the amount of stray light determined by the product of the respective reflectances of recording and reading layers decreases as the stray light is reflected a greater number of times. Accordingly, for practical purposes, considering stray light reflected off multiple surfaces three times is sufficient.
In the phenomena shown in FIGS. 15 to 18, the beams 70 and 71 have the same optical path length and the same radius of light flux if T1 is set to be equal to T2. In this case, the beams 70 and 71 enter the photodetector and are detected at the same time. Likewise, the beams 70 and 72 have the same optical path length and the same radius of light flux if the total of T1 and T2 is set to be equal to the total of T3 and TC. Also, the beams 70 and 73 have the same optical path length and the same radius of light flux if T3 is set to be equal to T1. Accordingly, making all interlayer distances different is a generally employed technique to avoid the confocal crosstalk.
Non-Patent Literature 1: Ichimura et. al., Appl. Opt, 45, 1974-1803 (2006), and Non-Patent Literature 2: K. Mishima et al., Proc. of SPIE, 6282, 628201 (2006) are introduced as the Prior Art Document.
In some cases, concavities and convexities for tracking control such as grooves and lands should be formed in each recording and reading layer. In these cases, concavities and convexities should be formed in each intermediate layer with a stamper, so that an error is likely to be generated in the film thicknesses of the intermediate layers. The intermediate layers may be set to have different film thicknesses in consideration of the effect of such an error generated during film deposition in advance. This however requires setting of a rather large difference between film thicknesses, resulting in more and more greater thickness of a multilayer optical recording medium.